1. Field of the Invention
The present invent ion relates to a semiconductor device which has a voltage sense function by which voltage between main electrodes can be measured by measuring smaller sense voltage.
2. Description of the Prior Art
FIG. 27 is a sectional view showing a configuration of a conventional voltage sense device. As shown in FIG. 27, an N.sup.- epitaxial layer 2 is formed on a P.sup.- substrate 1, and the N.sup.- epitaxial layer 2 is separated by a P diffusion separating region 3. An N.sup.+ buried region 4 is formed in part of an interface between the P.sup.- substrate 1 and the N.sup.- epitaxial layer 2, an N.sup.+ diffusion region 6 is formed extending from top of the N.sup.+ buried region 4 to a surface of the N.sup.- epitaxial layer 2, and a P diffusion region 5 and an N.sup.+ diffusion region 7 are selectively formed in the surface of the N.sup.- epitaxial layer 2.
Then, an electrode 21 is formed on the P diffusion separating region 3, a sense electrode 22 is formed on the N.sup.+ diffusion region 6, an electrode 23 is formed on the N.sup.+ diffusion region 7, and an electrode 24 is formed on the P diffusion region 5. Reference numeral 26 denotes an insulating film.
In such a configuration, when the electrode 21 and 24 are set at a ground level and voltage applied to the electrode 23 is gradually raised, depletion layers expand in PN junctions between the P.sup.- substrate 1 and the N.sup.- epitaxial layer 2 and between the P diffusion region 5 and the N.sup.- epitaxial layer 2, as shown in FIG. 28. Dot-and-dash lines in FIG. 28 show an expansion of the depletion layers.
At this time, the depletion layers expanding from the P diffusion region 5 and the P.sup.- substrate 1 towards the N.sup.- epitaxial layer 2 cause a 3unction type FET (JFET) of PN junction having a width X in FIG. 27 in the N.sup.- epitaxial layer 2 to be activated, and the electrode 23 and the sense electrode 22 assume an identical value in voltage until the depletion layers pinch off, but once they have pinched off as shown in FIG. 29, the depletion layers separate voltage at the electrode 23 from that at the sense electrode 22, which causes the sense electrode 22 no longer to rise. Dot-and-dash lines in FIG. 29 show an expansion of the depletion layers.
FIG. 30 is a graph showing sense voltage V22 at the sense electrode 22 related to voltage V23 applied to the electrode 23. As shown in FIG. 30, once the applied voltage V23 rises over pinch-off voltage VP and the depletion layers pinch off as shown in FIG. 29, the sense voltage V22 can scarcely rise even if the applied voltage V23 becomes larger. As a result, the applied voltage V23 at high level can be sensed by the measured voltage V22 at low level.
A conventional voltage sense device is configured as mentioned above, and its voltage sense characteristic depends upon a JFET configuration, where, for example, the voltage sense characteristic is determined by dimensions, X and Y, in FIG. 27 and a value of epitaxial resistivity in the N.sup.- epitaxial layer 2, and especially, the dimension Y and the epitaxial resistivity are values for determining the pinch-off voltage of the JFET.
However, it is difficult to accurately determine the dimension X because of mask shear and diffusion dispersion in forming the N.sup.+ buried region 4 and the P diffusion region 5. Also, it is difficult to accurately control an epitaxial resistivity of the N.sup.- epitaxial layer 2. Moreover, it is difficult to control a film thickness of the N.sup.- epitaxial layer 2, and it is also difficult to accurately determine the dimension Y because of diffusion dispersion of the P diffusion region 5 and the like.
Thus, in the conventional voltage sense device, there are variations in a variety of parameters for determining a voltage sense characteristic, and therefore, there is a problem that it is impossible to get a good voltage sense characteristic. Also, it is necessary to provide the N.sup.+ buried region 4 to obtain a JFET configuration, and there arises a problem that a manufacturing process is complicated.